Method of making flash memory cell with self-aligned tunnel dielectric area

ABSTRACT

This invention provides a stacked gate flash memory cell structure and a method for forming the stacked gate flash memory structure. The invention uses a large angle ion implant beam without wafer rotation to form the source and drain regions of the memory cell. A low doped region is formed between an edge of the first gate electrode and an edge of either the source or drain regions. The tunnel dielectric is formed directly above the low doped region. The width of the low doped region is controlled by the angle of the large angle ion implant beam and can be very accurately controlled. The tunnel dielectric is formed independently of the gate dielectric and the thickness of each can be optimized. The tunnel dielectric area can be made very small which improves reliability and reduces the voltage necessary to program and erase the memory cell.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a stacked gate flash memory cell structure andprocess which uses a large angle ion implant beam to form the source anddrain regions in the cell. A low doped region is formed between the edgeof either the source or drain and the edge of the first gate electrode.The tunnel dielectric is grown over this low doped region and isself-aligned with it. This method provides a self-aligned very smalltunnel dielectric area.

(2) Description of Related Art

Conventional stacked gate flash memory cell structures have thedisadvantage of a large tunnel dielectric area which requires largevoltages for programming and erase operations of the memory cell. Inaddition larger tunnel dielectric areas introduce more defects and lowerdevice yield. Often the tunnel dielectric is the same dielectric as thegate dielectric which leads to a compromise between gate dielectricthickness and tunnel oxide thickness.

This invention has the advantage of a tunnel dielectric which isindependent of the gate dielectric and the thickness of each can beoptimized. In this invention the tunnel dielectric is self-aligned tothe source and gate and has a width that can be accurately controlled.

SUMMARY OF THE INVENTION

It is a principle object of the invention to provide a method of forminga stacked gate flash memory cell with a smaller tunnel dielectric areathan can be achieved using conventional processing.

It is another object of the invention to provide a stacked gate flashmemory cell with smaller tunnel dielectric area than can be achievedusing conventional methods.

A smaller tunnel dielectric area requires a lower voltage forprogramming and erasing operations in the memory cell. A smaller tunneldielectric area also reduces defects and improves device yield.

These objectives are achieved by using a self-aligned method of formingthe tunnel dielectric region which permits the tunnel dielectric regionto be kept small while maintaining critical dimensional tolerances. Aoblique angle ion implant beam is used to form the N⁺ source and drainregions of the flash memory cell using the first gate electrode as amask. This results in either the source or drain region extending underone edge of the first gate electrode and a gap width between an edge ofeither the source or drain region, whichever does not extend under thefirst gate electrode, and an edge of the first gate electrode. Normallydirected ion implantation is then used to form an N⁻ low doped region inthe gap width. The width of the low doped region can be accuratelycontrolled by adjusting the angle of the oblique angle ion implant beam.There is no wafer rotation when the oblique angle ion implant beam isused.

A thermal oxide is then grown which will become the tunnel dielectric ortunnel oxide region and a self-aligned thick oxide region. The thermaloxide grows faster over the N⁺ source and drain regions than over the N⁻light doped region. The tunnel oxide is the thin oxide formed over thelight doped region. The width of the tunnel oxide is controlled by thewidth of the light doped region. The width of the light doped region isaccurately controlled by the angle of the large angle ion implant beam.The accuracy with which the width of the tunnel oxide region can becontrolled and the self-aligned nature of the tunnel oxide region areboth important in achieving a very small tunnel oxide width.

The flash memory cell uses a floating gate and a control gate structure.When a suitable potential is applied to the control gate, which is alsothe Word line, while grounding the source or drain, whichever is formedadjacent to the light doped region, electrons are injected into thefloating gate from the source or drain, whichever is formed adjacent tothe light doped region, through the tunnel oxide. When a suitablepotential is applied to the source or drain, whichever is formedadjacent to the light doped region, while grounding the control gateelectrons are injected from the floating gate into the source or drain,whichever is formed adjacent to the light doped region, through thetunnel oxide. Smaller tunnel oxide areas require smaller voltages forthe programming and erase operations just described.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross section view of the flash memory cell after formationof the first gate electrode with a nitride layer.

FIG. 2A is a cross section view of the flash memory cell after formationof the first gate electrode with the nitride layer showing the largeangle ion implant beam used to form the source and drain regions.

FIG. 2B is a cross section view of the flash memory cell showing theformation of the source and drain regions using the large angle ionimplant beam.

FIG. 3A is a cross section view of the flash memory cell showing theformation of the light doped region using normally directed ionimplantation.

FIG. 3B is a top view of the flash memory cell after formation of thesource, drain, and light doped regions.

FIG. 4 is a cross section view of the flash memory cell showing theformation of the tunnel oxide, self-aligned thick oxide, and sidewalloxide regions.

FIG. 5 is a cross section of the flash memory cell after the second gateelectrode material has been formed.

FIG. 6A is a cross section view of the flash memory cell after thesecond gate electrode has been formed, the oxide/nitride/oxide layer hasbeen formed, and the control gate electrode or word line has beenformed.

FIG. 6B is a top view of the flash memory cell showing the first gateelectrode, the second gate electrode, the control gate electrode, thesource, the drain, and the light doped regions.

FIG. 7 is a cross section view of the flash memory cell after theinsulating dielectric layer and contacts to the control gate electrodehave been formed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer now to FIG. 1 through FIG. 7, there is shown the principleembodiment for the method of forming the flash memory cell withself-aligned tunnel dielectric area. FIG. 1 shows a P type siliconsubstrate 20 with a layer of gate oxide 30, such as SiO₂, with athickness of between about 200 Angstroms and 1000 Angstroms formed onthe silicon substrate 20. A polysilicon first gate electrode 40 isformed on the layer of gate oxide and a nitride layer 42 is formed onthe first gate electrode. At this stage of the process the first gateelectrode 40 extends over a number of adjacent memory cells in order toserve as a mask for the formation of the source, drain, and light dopedregions. The first gate electrode will be modified later to be isolatedto a single cell. The first gate electrode and nitride layer are formedby means of conventional deposition, photoresist, lithography, andetching methods. Examples of these conventional methods can be found inthe book "VLSI PROCESS TECHNOLOGY" Second Edition, by S. M. Sze,published by McGraw-Hill Book Co., New York, N.Y., 1988, pages 221-226and 223-245.

This embodiment describes a stacked gate flash memory cell where thedrain region extends under an edge of the first gate electrode and a gapwidth is formed between the edge of the source region nearest the firstgate and the edge of the first gate electrode nearest the source region.The invention works equally well if the source and drain regions areinterchanged.

Next, as shown in FIG. 2A, a large angle ion implant beam 50 is used toform the source and drain regions. The oblique angle ion implant beamforms an angle 52 of between about 10° and 60° between the beamdirection and the sidewall of the first gate electrode. As shown in FIG.2B N⁺ source 22 and drain 24 regions are formed using a large angle ionimplant beam 50 of arsenic ions with between about 1×10¹⁵ and 8×10¹⁵ions/cm² at between about 30 and 100 keV. There is no wafer rotationduring this implantation step. As shown in FIG. 2B, one edge of thedrain 24 region extends under the edge of the first gate electrode 40nearest the drain 24 region. There is a gap width 56 between the edge ofthe first gate electrode 40 nearest the source 22 region and the nearestedge of the source 22 region. The gap width 56 is the tunnel oxidedimension and is between about 0.1 microns and 0.3 microns. The gapwidth 56 is very nearly equal to the height 54 of the first gateelectrode 40 with the nitride layer 42 multiplied by the tangent of theangle 40 between the oblique angle ion beam 50 and the sidewall of thefirst gate electrode 40.

Next, as shown in FIG. 3A, the gate oxide layer not covered by the firstgate electrode is removed by etching using the first gate electrode 40with the nitride layer 42 as a mask. Next, the N⁻ light doped region 26is formed between the edge of the first gate electrode 40 nearest thesource 22 region and the nearest edge of the source 22 region. The lightdoped drain region is formed using a normally directed ion beam 52 ofphosphorous or arsenic with between about 1×10¹³ and 1×10¹⁴ ions/cm² atbetween about 30 and 50 keV. The edge of the first gate electrode 40serves as a mask to determine one edge of the light doped region. FIG.3B shows the top view of the flash memory cell at this stage offormation showing the first gate electrode 40, the source region 22, thedrain region 24, and the light doped drain region 26.

Next, as shown in FIG. 4, the side wall oxide 34, the tunnel oxide 32,and the self-aligned thick oxide 36 are formed using thermal oxidationat between about 800° C. and 950° C. The tunnel oxide 32 is betweenabout 60 Angstroms and 100 Angstroms thick. The self-aligned thick oxideis grown at the same time as the tunnel oxide but since it is formed onan N⁺ region the self-aligned thick oxide grows faster than the tunneloxide which is formed on an N⁻ region. The self-aligned thick oxidethickness is between about 200 Angstroms and 500 Angstroms.

The tunnel oxide and the self-aligned thick oxide are formedindependently of the gate oxide so that the thickness of the gate oxideand the tunnel oxide can both be optimized. Since the self-aligned thickoxide and the tunnel oxide regions are determined by the material theyare grown on the self-aligned thick oxide is self aligned to the tunneloxide and the width of the tunnel oxide region is automaticallycontrolled.

Next, as shown in FIG. 5, the nitride layer is removed from the firstgate electrode and a second polysilicon layer 46 is formed on thesilicon substrate using conventional means. The second polysilicon layer46 forms electrical contact with the first gate electrode 40. Next, asshown in FIG. 6A, the second gate electrode 46 is formed by patterningthe second polysilicon layer using conventional means of photoresist,lithography and etching. The first gate electrode 40 is electricallyshorted to the second gate electrode 46 and together they will form thefloating gate.

Next a layer of oxide/nitride/oxide 47 with an effective thickness ofbetween about 100 Angstroms and 300 Angstroms is formed over the secondgate electrode by means of conventional chemical vapor deposition and/orthermal oxidation methods. Next a control polysilicon layer is formed onthe layer of oxide/nitride/oxide 47. The control gate electrode or wordline 48 is then formed by patterning the control polysilicon layer usingconventional means of photoresist, lithography and etching. The floatinggate is formed from the electrically shorted first gate electrode andsecond gate electrode and must not extend from one cell to another butmust be isolated to a single cell. The etching of the controlpolysilicon layer to form the control gate electrode or word line iscontinued through the first and second polysilicon layers to achievethis isolation. Examples of such conventional means are given in thepreviously cited book by Sze, pages 221-226 and 233-245. FIG. 6B showsthe top view of two adjacent flash memory cells showing the word lines48 at right angles to the source region 22, the drain region 24, and thelight doped region 26; and the separation of the floating gate betweencells.

Next, as shown in FIG. 7, the flash memory cell is completed by formingan insulation layer such as borophosphosilicate glass 70 over thesilicon substrate. Contact openings are formed in the insulation layerto form metal contacts 72 to the word lines 48. A patterned metalconductor layer and a passivation dielectric layer, not shown, areformed on the silicon substrate to complete the formation of the stackedgate flash memory cell device. These completion operations can beaccomplished using conventional means.

Refer now to FIG. 6B and FIG. 7, there is shown an embodiment of theflash memory cell with a self aligned tunnel dielectric area. FIG. 7shows a cross section view of the flash memory cell. The P type siliconsubstrate 20 has N⁺ source 22 and drain 24 regions formed therein. Oneedge of the drain 24 region extends under the polysilicon first gateelectrode 40. This embodiment describes a stacked gate flash memory cellwhere the drain region extends under an edge of the first gate electrodeand a gap width is formed between the edge of the source region nearestthe first gate and the edge of the first gate electrode nearest thesource region. The invention works equally well if the source and drainregions are interchanged. There is an N⁻ light doped region 26 betweenthe edge of the source 22 region nearest the first gate electrode 40 andthe edge of the first gate electrode. There is a tunnel oxide area 32,such as SiO₂ with a thickness of between about 60 Angstroms and 100Angstroms, formed directly over the light doped region 26. Aself-aligned thick oxide 36, such as SiO₂ with a thickness of betweenabout 200 Angstroms and 500 Angstroms, is formed directly over thesource 22 region and that part of the drain 24 region which is not underthe first gate electrode. A gate oxide 25, such as SiO₂ with a thicknessof between about 200 Angstroms and 1000 Angstroms, is formed on thesilicon substrate and a polysilicon first gate electrode 40, with athickness of between about 1000 Angstroms and 5000 Angstroms, is formedon the gate oxide 30. A polysilicon second gate electrode 46, with athickness of between about 500 and 2000 Angstroms, is formed over thefirst gate electrode 40 after the tunnel oxide 32 and self-aligned thickoxide 36 have been formed and extends over the tunnel oxide 32 and overa portion of the self-aligned thick oxide 36 on both sides of the firstgate electrode 40. The second gate electrode 46 makes electrical contactwith the first gate electrode 40. A sidewall oxide 34 is formed on thesidewalls of the first gate electrode. A layer of oxide/nitride/oxide 47with a thickness of between about 100 and 300 Angstroms is formed on thesecond gate electrode 46. A polysilicon control gate electrode 48, witha thickness of between about 2000 Angstroms and 5000 Angstroms, isformed over the oxide/nitride/oxide 47 layer. The control gate electrode48 is the word line.

FIG. 6B shows a top view of the flash memory cell. The word line 48 isat right angles to the source 22, the drain 24, and the light dopeddrain region. Refer again to FIG. 7, an insulating dielectric layer 70,such a borophosphosilicate glass, is formed over the silicon substrateafter the word lines 48 have been formed and metal contacts 72 areformed through the insulating dielectric layer 70 to the word line 48.No electrical contact is made to the floating gate, formed from theelectrically shorted first gate electrode 40 and second gate electrode46, and the floating gate for each stacked gate flash memory cell isseparate from the floating gates for other stacked gate flash memorycells. A patterned conducting metal layer and passivation dielectriclayer, not shown, complete the formation of the flash memory cell.

Referring again to FIG. 7, the first gate electrode 40 and the secondgate electrode 46 are electrically connected together and form thefloating gate of the flash memory cell. The control gate electrode 48forms the word line. When a suitable potential is applied to the wordline 48 while grounding the source 22 electrons are injected into thefloating gate, first gate electrode 40 and second gate electrode 46,from the source 22 through the tunnel oxide 32. When a suitablepotential is applied to the source 22 while grounding the word line 48electrons are injected from the floating gate, first gate electrode 40and second gate electrode 46, into the source 22 through the tunneloxide 32. Smaller areas of the tunnel oxide region require lowervoltages for the writing and erasing operations just described.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of forming an improved memory cell,comprising the steps of:providing a semiconductor substrate; forming agate dielectric layer on the surface of said semiconductor substrate;forming a layer of first gate electrode material on the surface of saidgate dielectric layer; forming a nitride layer on the surface of saidlayer of first gate electrode material; forming a photoresist layer onthe surface of said nitride layer; forming a first gate electrodepattern in said photoresist layer; forming a first gate electrode withsidewalls and a top surface nitride layer by means of etching said firstgate electrode pattern in said layer of first gate electrode materialand said nitride layer using said first gate electrode pattern in saidphotoresist layer as a mask; forming heavily doped source and drainregions in said semiconductor substrate by means of a oblique angle ionimplantation; etching said first gate electrode pattern in said gatedielectric layer using said first gate electrode with sidewalls and atop surface nitride layer as a mask; forming a lightly doped region insaid semiconductor substrate using normally directed ion implantation;forming a sidewall oxide layer on the sidewalls of said first gateelectrode; forming a tunnel oxide layer over said lightly doped regionin said semiconductor substrate; forming a self-aligned oxide layer oversaid source and drain regions in said semiconductor substrate; removingsaid top surface nitride layer from said first gate electrode withsidewalls; forming a layer of second gate electrode material on thesurface of said semiconductor substrate after said first gate electrodehas been formed and said top surface nitride layer has been removed sothat said first gate electrode makes electrical contact with said layerof second gate electrode material; forming a second gate electrode byetching a second gate electrode pattern in said layer of second gateelectrode material using photoresist masking and lithography so thatsaid second gate electrode makes electrical contact with said first gateelectrode; forming a layer of oxide/nitride/oxide on the surface of saidsecond gate electrode; forming a layer of control gate electrodematerial on said layer of oxide/nitride/oxide; forming a control gateelectrode by etching a control gate electrode pattern in said layer ofcontrol gate electrode material using photoresist masking andlithography; forming a floating gate from said first gate electrode andsaid second gate electrode; forming an insulating-dielectric layer onthe surface of said semiconductor substrate after said control gateelectrode has been formed; forming contact openings in said insulatingdielectric layer; forming metal contacts in said contact openings insaid insulating dielectric layer; forming a patterned metal conductorlayer on the surface of said insulating dielectric layer; and forming apassivation dielectric layer on the surface of said insulatingdielectric layer after, said patterned metal conductor layer and saidmetal contacts have been formed.
 2. The method of claim 1 wherein saidsemiconductor substrate is a P conductivity type silicon substrate. 3.The method of claim 1 wherein said layer of first gate electrodematerial is polysilicon with a thickness of between about 1000 Angstromsand 5000 Angstroms.
 4. The method of claim 1 wherein said layer ofsecond gate electrode material is polysilicon with a thickness ofbetween about 500 Angstroms and 2000 Angstroms.
 5. The method of claim 1wherein said layer of control gate electrode material is polysiliconwith a thickness of between about 2000 Angstroms and 5000 Angstroms. 6.The method of claim 1 wherein said gate dielectric layer is SiO₂ with athickness of between about 200 Angstroms and 1000 Angstroms.
 7. Themethod of claim 1 wherein said oblique angle ion implant beam forms anangle of between about 10° and 60° with a line normal to the surface ofsaid semiconductor substrate.
 8. The method claim 1 wherein there is norotation of said semiconductor substrate when the source and drainregions are being formed by means of said oblique angle ion implantbeam.
 9. The method of claim 1 wherein said large angle ion implantationuses arsenic with a dose of between about 1×10¹⁵ and 8×10¹⁵ ions/cm² atbetween about 30 and 100 keV.
 10. The method of claim 1 wherein saidnormally directed ion implantation uses phosphorous or arsenic with adose of between about 1×10¹³ and 1×10¹⁴ ions/cm² at between about 30 and50 keV.
 11. The method of claim 1 wherein said tunnel oxide layer andsaid self-aligned oxide layer are formed using the same thermaloxidation at between about 800° C. and 950° C.
 12. The method of claim11 wherein said tunnel oxide layer is between about 60 and 100 Angstromsthick.
 13. The method of claim 11 wherein said self-aligned oxide layeris between about 200 and 500 Angstroms thick.
 14. The method of claim 11wherein the width of said tunnel oxide layer is between about 0.1 and0.3 microns.